The present invention relates to automatic shifting speed change transmissions employed in motor vehicles where electrically operated solenoid valves are controlled by an electronic computer for controlling the flow of pressurized fluid to transmission shift actuators such as band clutch actuators employed in the transmission shifting operation. The electronically controlled solenoid operated valves provide improved shifting capabilities for the transmission as compared to hydro-mechanical shift controls; and, in particular, permits staged or progressive release and application of the band clutches for effecting smoother speed changes in the transmission.
Such valve assemblies are, in current production transmissions, mounted internally on the transmission valve body and supplied with pressurized fluid from a pump disposed in the transmission.
Heretofore, in controlling the shifting of an automatic speed change power transmission it has been known to use a pilot operated valve to control flow to a large flow control valve for clutch actuation such as shown in FIG. 1. FIG. 2 uses a VFS (variable force solenoid) without need of a flow control valve.
It is also known to use solenoid valves controlled by an electronic controller and to sense line pressure in order that the controller can regulate line pressure to a desired valve. An example of this latter type control is that shown and described in U.S. Pat. No. 4,781,080 which teaches a pressure sensor for providing a control signal to an electronic controller for operating a solenoid valve to regulate the supply pressure from the pump to a primary regulator valve.
Heretofore, in automatic shifting speed power transmissions for motor vehicles, with the advent of electronic control it has been desired to provide some power transmission during speed changes. Thus, the rate of engagement and disengagement of the individual clutch bands was controlled to permit speed changes prior to complete deactuation of the driving clutch. However, this required a very high degree of calibration of the solenoid operated valve controlling the pressure to the clutch actuator and has proven to be difficult to achieve reliably.
The primary technique employed in these prior art systems for shift control has been the use of an algorithm in the electronic controller utilizing a known calibration curve for the solenoid valves to yield the desired pressure output based upon the current input to the valve from the controller. However, it has been found in service that the ability to calibrate and maintain the calibration of the solenoid valves to provide sufficiently accurate control of the output pressure to the shift actuators has been limited. The pressure output of the valves has shifted with temperature variations and exposure to the transmission fluid contamination and viscosity changes.
Thus, it has long been desired to provide a way or means of providing closed loop control of solenoid operated valves for shift clutch actuator control in an automatic speed change transmission which would obviate the need for accurate flow calibration of the solenoid valves. Furthermore, it has been desired to provide such a way or means of closed loop shift control which is low in cost and easy to manufacture for a high volume motor vehicle transmission such as employed in passenger cars and light trucks.
Referring to FIG. 3, the aforesaid prior art system is shown in diagrammatic form wherein a pressure signal is provided to the transmission control unit (TCU) which controls the solenoid valve providing pump output to a line pressure regulator.
In the prior art, it is known, as for example in U.S. Pat. No. 4,776,233, to detect the torque transmitted by the clutch by disposing a pair of speed sensors on the input and output members of each speed change gear set where the speed difference between the input and output member was used as an analog of torque transmitted to provide a feedback signal to the controller for the solenoid operated valve. However, as the speed sensors must be mounted on each gear set, this has been costly and difficult particularly where planetary gear sets are employed. Thus, it has been desired to provide a simple way or means of providing closed loop control with a feedback signal indicative of the torque transmitted by a particular shifting clutch band.
The present invention provides closed loop pressure control of hydraulically operated shift clutch actuators in an automatic speed change power transmission where the shift clutch actuators receive pressure from the outlet of an electronically controlled solenoid operated valve. The present invention is particularly applicable to automatic speed change transmissions having hydraulic actuators for operating band clutches for effecting the speed changes.
A pressure sensor detects the pressure supplied to each of the hydraulic clutch actuators such as hydraulic piston actuators for band clutches and the sensed pressure is supplied to a transmission electronic controller (TCU) which compares the pressure with a command pressure signal sent to the transmission from the vehicle powertrain electronic controller. The current to each solenoid valve may then be varied until the difference between the command pressure signal and the sensed pressure from the hydraulic clutch actuator is sufficiently near zero as to be considered negligible and indicating a match between the command signal and the actual clutch pressure. The present invention thus provides rapidly responsive shift control inasmuch as a pressure sensor may be employed with each band clutch hydraulic actuator and the full range of speed changes for the transmission may be programmed electronically to vary the speed of the clutch engagement or disengagement as desired utilizing the clutch band actuator pressure as an analog of the torque transmitted by the clutch. The present invention provides more accurate control of the engagement and disengagement of each shift clutch and this improves the ability to change speed smoothly and to maintain power transmission during speed changes.